1. Field of the Invention
This invention relates to refrigeration apparatus, and in particular, to means for automatically controlling defrosting of such apparatus.
2. Description of the Prior Art
United States Letters Patent pertinent to the present invention include:
2,229,612 Pearce Refrigerating Apparatus 2,295,570 Dunmore Humidity Variable Resistance 2,424,735 Boothroyd Humidity Control Apparatus 2,444,030 Burch Humidity and Temperature Responsive Device 2,510,018 Gillingham Electrolytic Humidostat 2,927,524 Gregor Polyelectrolyte Water Indicating Devices 3,029,610 Armentrout Refrigerating Apparatus Including Defrosting Means 3,120,108 Pansing Refrigerating Apparatus Including Defrost Control 3,335,576 Phillips Defrost Control for Refrigeration Apparatus 3,460,352 Lorenz Defrost Control 3,474,638 Dodge III Electronic Refrigeration System Defrost Control 3,622,523 Amin Air Fireable Compositions Containing Vanadium Oxide and Boron, and Devices Therefrom
It is conventional to provide means for automatically defrosting refrigeration systems utilizing evaporator-type cooling means, such as in refrigerators, freezers, and the like. One means for controlling the defrosting operation therein has been a clock-type control which has the disadvantage of requiring the setting thereof for worst case climate and usage conditions so that often too many defrost periods of too long a duration result causing unnecessary power consumption and food thermal cycling. Such clock controls have been found to require substantial maintenance and create a potential noise problem.
Other prior art demand defrost controls have proven to be unreliable and costly. One such control utilizes a hygrometer for sensing the humidity condition of the refrigerated air and an E-Cell coulometer for integrating the humidity information. Such coulometers permit resetting the stored charge thereof for recycling the control during the defrost operation. The electrical circuitry of such controls is relatively complex and costly.
Another prior art control utilizes a photoelectric means for detecting the presence of frost. Such controls have been found to require substantial maintenance. Further, light energy from the lamp tends to heat and distort the formation of frost in the area being monitored.
Another prior art control utilizes one or more thermistor type thermal responsive devices to sense the presence of frost at specific points in the evaporator. It has been found, however, that sensing frost at a given point is unreliable as frost may form irregularly on the evaporator.
Thermal responsive devices have been utilized in connection with defrosting means for controlling termination of the defrost operation. Typically, a bimetal thermostat may be mounted in thermal transfer association to the evaporator and wired in series with the defrost heater to open the heater circuit when the evaporator reaches a high temperature. A clock has been conventionally provided therewith for timing a remainder portion of the defrost period. This remaining period, known as the run-off period, allows melted frost to drain away. One disadvantage of the above system is that the run-off period normally must be longer than that necessary to complete an optimum draining operation. However, the above system has the safety advantage that both the clock and the bimetal thermostat must fail before the refrigeration system would be subjected to unduly high temperatures.